In the field of a photomechanical process, photographic materials having good capability to reproduce originals, stabilized processing solutions and simplified replenishment have been required to cope with variety and complexity of printing matter.
Particularly, line originals used for a line camera work are usually made by pasting phototypesetting letters, handwriting letters, illustrations, halftone photographs, and so forth. That is, images which are different in density and line width coexist in an original. Therefore, process cameras, photographic materials, and methods of image formation to make it possible to faithfully reproduce such an original have been urgently demanded. On the other hand, enlargement (spread) or reduction (choke) of halftone photographs is widely carried out for platemaking for catalogs or large-sized posters. The platemaking involving the enlargement of halftone dots brings about coarse screen ruling, resulting in photographing of out-of-focus dots, whereas the reduction brings about minute screen ruling per inch, resulting in photographing smaller dots. Methods of image formation which have a broader latitude has been accordingly required to maintain the capability to reproduce halftone gradation.
A method for obtaining a line drawing or halftone image having high contrast and high blacking density in which the difference between image areas and nonimage areas are clearly distinguished by processing a silver halide photographic material of a lithographic type containing at least 50% or more of silver chlorobromide with a hydroquinone-based developer having a very low effective concentration of sulfite ion (usually 0.1 mol/liter or less) is known as a system satisfying the demand for the broader latitude. However, in this method, the developer is very unstable to air oxidation, and various attempts have been made to keep stable activity of the developer on continuous use.
To solve such instability in image formation, a system of image formation which can ensure superhigh-contrast photographic characteristics by developing with a processing solution having good storage stability have been required. One of such systems is proposed in U.S. Pat. No. 4,166,742, which can form a superhigh-contrast negative image having .gamma. exceeding 10 by processing a silver halide photographic material of a surfacelatent image type containing a certain acylhydrazine compound with a developer of pH 11.0 to 12.3 which contains sulfite ion as a preservative. This system of image formation has performances excellent in sharpness of halftone quality, processing stability, rapidity, and capability to reproduce an original.
However, the developer used for this system of image formation has the disadvantage of being liable to fog, because the developer has to keep relatively high pH to obtain high contrast. To control the fogging, a technique for improving significantly image quality by adding a redox compound which is oxidized to release a development inhibitor is disclosed in JP-A-61-213847 (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
On the other hand, some kinds of merocyanine dyes are known to be used as spectral sensitizing dyes for photographic emulsion as disclosed in U.S. Pat. Nos. 3,480,439 and 3,625,698, and so forth. Examples of sensitizing dyes having characteristics which give a hard and sharp halftone image include dimethinemerocyanine dyes containing a thiohydantoin ring substituted by a pyridyl group disclosed in JP-A-55-45015 corresponding to U.S. Pat. No. 4,266,003; and dimethinemerocyanine dyes containing a thio-hydantoin ring substituted by a phenyl group disclosed in JP-B-54-34532 (The term "JP-B" as used herein means an "examined Japanese patent publication"). However, the silver halide photographic materials containing such merocyanine dyes, in fact, have the disadvantage of low sensitivity, low contrast and deteriorated storage stability, or of being liable to leave color on the materials after being processed. Thus, the advent of spectral sensitizing dyes which do not have such drawbacks has been urgently expected.